The Raman spectra of two critical solution mixtures, namely cyclo-hexane-aniline and iso-butyric acid-water, are obtained and compared with the spectra of the component liquids. No changes are observed in the cyclohexane-aniline mixture. In the iso-butyric acid-water mixture the following changes are observed: Three fairly intense lines at 510, 1430 and 1654 cm.-1 of the acid disappear in the mixture and instead, broad lines appear at 519 and 1715. The strongest line of the acid at 800 cm.-1 becomes slightly broader and less intense. The strong water band at 3460 appears to be shifted to a slightly higher frequency. From a comparison of these changes with those reported for formic, acetic and trichloroacetic acids, it is concluded that at the critical temperature, the acid molecules change over from dimer to monomer state. This indicates that at or above the critical temperature, the two liquids in the mixture become homogeneous as in solutions. The results are discussed in relation to the cluster formation in the binary liquid mixtures.